1. FIELD OF THE INVENTION
The present invention relates to a particular interferometric system and a device for detecting and measuring the concentration of a gas in a gaseous mixture, the absorption structure of said gas being quasi-periodic in a determined spectral band. The devices provided herein make it possible to use materials with slight birefringence and in particular are intended for analysis of gases in the infra-red.
1. DISCUSSION OF RELATED ART
Measurement of the concentration of a gas by the study of its absorption spectrum has been the object of several patents which are concerned with interferometric devices which utilize Fourier formalisms.
For a gas exhibiting locally, in a band of the scale of wave numbers, a quasi-periodic absorption spectrum of period p, the Fourier transform of the spectrum produced by an interferometric system placed in the path of a light ray, which has gone through a mixture containing the gas, exhibits a measurable maximum on the signal transmitted by the interferometric system for .DELTA..sub.c =1/p, .DELTA..sub.c being the path difference characterizing said system. This maximum reflects the similarity of the fringe system produced by the interferometric system to the fringe system of the absorption spectrum of the gas.
French patents published under numbers 2,340,540; 2,555,748; 2,566,748; 2,566,532 and 2,581,190 describe interferometric devices for detecting a gas in a mixture. These devices, in their simplest form comprise as an interferometer: a plate of birefringent material, of birefringence .DELTA..sub.n and thickness e, cut for example, parallel to the axis of the material and providing a path difference .DELTA.=e=.DELTA..sub.n. One of the simplest devices is described in the publication FR. A.2,340,540 and comprises successively along the same optical axis: a light source with continuous emission spectrum; a lens; a gas cell containing the mixture to be analyzed; a filter isolating the absorption band of the gas studied; a polarizer; a birefringent plate; an analyzer; a lens; a diaphragm located at the image focus of said lens; and a detector transforming the light energy into an electric signal, for example, a photoelectric multiplier, with which the interference-phenomenon is studied. The thickness of the birefringent plate, which is adapted to the gas studied, is selected to obtain behind the lens an order of interferences corresponding to a maximum of illumination of the interference phenomena in the presence of the substance studied.
For an easy and precise reading of the output signal, the luminous flux is modulated by rotation of the polarizer or analyzer, to form alternately in the focal plane of the lens, a given fringe system or its complementary, when the polarizer and analyzer are parallel or perpendicular. The rotation of the analyzer or polarizer is made at frequency f, so that the interference term is modulated and the signal delivered by the detector exhibits a double frequency 2f. The amplitude of this output signal is proportional to the concentration of the gas studied, which is circulating in the cell placed in front of the interferometer.
The advantage of the above devices resides in their small size, their very great stability and the ease with which signals can be modulated, either by the rotation of the analyzer and polarizer, or else by rotation of a plate .lambda./2, or by use of a photoelastic modulator, or by vibration of a periodic grating.
The above devices' drawbacks reside in the fact that in the spectral regions studied, (i.e., in the region that comprises the absorption region of the gas or gases studied), it is not always easy to find materials for making the polarizers and analyzers and the birefringent plate, that allow the analyzer to be made at a low cost. This is thought particularly true in the case where the region studied is an infrared region between 2 microns and 8 microns.
In such an infrared region (i.e., 2-8 microns), the making of appropriate polarizers and analyzers can also pose problems. Absorption of films polarizing infrared does not allow for going beyond 2.5 microns. For example, calcite cannot be used beyond 2.5 microns because it becomes dichroic. Dichroic synthetic materials are known in business under the name of POLAROID.RTM., and are currently used in the visible and ultraviolet light regions, but cannot be used in the infrared light region A solution which allows one to go beyond 2.5 microns is given in French patent 2,581,190, the solution consists of resorting to polarizing prisms of the Wollaston type of quartz or magnesium fluoride. The main drawbacks of these polarizers are their high cost and the slight light range that they allow.
Still, more acute problems can arise in the making of a birefringent plate useful in the study of the infrared region. The following serves as an example of such problems. In the following example, it is assumed carbon monoxide (CO) is in the gas mixture analyzed.
To make a birefringent plate that can be used in the infrared, two materials in particular can be used: rutile with a birefringence .DELTA..sub.n =0.2, and magnesium fluoride with a birefringence .DELTA..sub.n =10.sup.-2. At 4.6 microns CO exhibits an absorption band with a fine absorption structure with periodicity 67 .sigma.=.sup.2 cm.sup.-1. The thickness of the plate necessary in this case is e.sub.o, so that e.sub.o .DELTA..sub.n =1/.delta..sigma. which gives a thickness equal either to 500 mm in the case of the magnesium fluoride plate, which is impossible to achieve, or to 25 mm in the case of the rutile plate, which is impossible to envisage in ,industry because of the prohibitive cost associated with the production of a plate of such a thickness.
Thus, it is the fact that the required thicknesses and costs are too great and/or the fact that the birefringence of the materials usable (for example in the infrared) is too slight, which has led to the impossibility of making an interferometric analysis device using a birefringent plate as briefly described above and described in greater detail in the French patent 2,340,540.